1381 Update docker tag automatically on master

[RegisGraptin]

Following this issue https://github.com/drand/drand/issues/1381 we wanted to update automatically the tag reference in the 'docker/README.md' file.

Notes:

• I am also wondering if we should not also modify other files. See some reference in:
  • 'docker/start-network-with-nginx.sh'
  • 'docker/docker-compose-network.yml'
  • 'docker/docker-compose-nginx.yml'
  • 'docker/docker-compose.yml'

It seems it refers to 'ghcr.io/drand/go-drand' or 'ghcr.io/drand/go-drand-local'.